What are arthropod-borne illness and disease?
Arthropod-borne illnesses are diseases that are spread by arthropods (insects) and are commonly seen in tropical and subtropical climates. The phylum Arthropoda, a term that comes from arthro (meaning “joint”) and poda (meaning “foot”), is the largest phylum in the animal kingdom. This phylum consists of invertebrates (animals that lack a backbone) that manifest bilateral symmetry (in which both halves of the body are identical), an exoskeleton (an external skeleton), a segmented body (a body divided into sections), and jointed legs.
The most diverse category in the Arthropoda phylum is class Insecta, which includes some well-known disease carriers: mosquitoes, ticks, and flies. Other arthropods are mites, fleas, and lice.
According to the World Health Organization (WHO), one in every six persons has an arthropod-borne illness at any given time. Rarely are these diseases caused by the arthropod itself; rather, they are typically caused by pathogenic bacteria, viruses, and protozoa that are carried by a vector. Any arthropod that carries disease-causing microbes is called a vector or carrier.
Pathogenic microbes that move from an infected host to a healthy host through insect bites include bacteria, viruses, protozoa, and helminths. Examples of insect-borne illnesses that are caused by bacteria include Lyme disease, plague, and tularemia. Common insect-borne diseases that are of viral origin include West Nile encephalitis, Chikungunya, yellow fever, and dengue fever. These viruses typically belong to the arbovirus (arthropod-borne viruses) family. Parasitic protozoa cause diseases such as leishmaniasis, Chagas’ disease, and malaria. Helminths cause diseases such as ascariasis and lymphatic filariasis.
A vector can be used by a pathogen in many different ways. Sometimes the vector simply offers a means of mechanical transfer from one point to another, such as when a housefly picks up a pathogen from a garbage bin and then deposits the pathogen on food that is consumed by humans. In other instances, such as in the case of malaria, the malarial parasite Plasmodium multiplies in the gut of the female Anopheles mosquito (a vector) before being transmitted to a healthy host by an insect bite (via the mosquito’s saliva). Thus, in this case the vector serves as an intermediate host required to complete the life cycle of the pathogenic protozoan Plasmodium. The pathogen also can be deposited on the host’s skin through the insect’s feces, which then enters the host either through the bite site or through another open wound, as is the case in Chagas’ disease.
Persons who are not native to high-risk regions of the world, namely tropical regions, are considered most at risk because they have no common knowledge of these illnesses and have not become immune to secondary infections. Another risk factor is staying in the adobe houses used by locals; some vectors, such as the Triatomine bugs (also called kissing bugs) that transmit Chagas’ disease, for example, live in the mud walls and thatched roofs of rural houses in tropical regions.
Symptoms depend on the disease origin (bacterial, viral, protozoan, or helminthic).
Bacterial origin. Lyme disease is the most common tickborne disease in the United States and is caused by a corkscrew-shaped bacterium called Borrelia burgdorferi. Early symptoms of Lyme disease include one or more of the following: a characteristic skin rash called erythema migrans, tiredness, headache, chills, muscle and joint pain, and inflammation of the lymph glands. If the early symptoms are not pronounced or go untreated for any reason, later symptoms include arthritis (inflammation and pain in joints, which can become chronic), nervous system problems such as meningitis (marked by a fever, a stiff neck, and a severe headache), and Bell’s palsy (paralysis of facial muscles).
Another bacteria-caused arthropod-borne disease is plague, one of the most ancient diseases known to affect humans (an epidemic of the fourteenth century wiped out about one-third of the human population). It is caused by the fleaborne bacteria Yersinia pestis.
There are two kinds of plague: bubonic and pneumonic plague. Bubonic plague produces symptoms such as high fever, headache, chills, and painful swollen lymph nodes (also known as buboes). Pneumonic plague symptoms include fever, headache, and pneumonia-like symptoms such as shortness of breath, cough, chest pain, and blood in the sputum.
Tularemia, another disease of bacterial origin, produces signs and symptoms based on the portal (path) of entry of the pathogen. The most common form of tularemia is spread by tick or deer fly bites and produces high fever accompanied by skin ulcers and inflammation (pain and swelling) of the lymph nodes. If left untreated, the infection can become more severe and cause pneumonia-like symptoms such as chest pain, cough, and difficulty in breathing.
Viral origin. West Nile encephalitis is caused by the West Nile encephalitis virus and is classified as a neuroinvasive disease because several forms of this disease affect the nervous system. About 80 percent of people who are exposed to this virus are asymptomatic. However, in persons without symptoms, the disease typically starts with fever, headache, and chills, which can lead to convulsions, neck stiffness, and even paralysis.
Yellow fever, another arthropod-borne illness of viral origin, is named for the yellowish skin coloration that is commonly associated with the better-known liver disease called jaundice. Most yellow fever infections are relatively mild and can remain undetected. However, about 15 percent of persons exposed to the yellow fever virus face a severe life-threatening condition characterized by fever, aches, nausea, abdominal pain, and vomiting that later progresses to melena (blood in the stool), hematemesis (vomiting blood), jaundice, and coma.
Chikungunya, another arthropod-borne illness, was first reported in the mid-twentieth century and was seen primarily in the developing world; hence, very little is known about this disease. The disease has reemerged in more temperate climates, such as Italy and France. Chikungunya resembles dengue fever, in that they both produce symptoms such as high fever, rash, and arthralgia (pain in the joints). Even though most of the symptoms of Chikungunya recede with time, the arthralgia often becomes chronic or relapses frequently after short breaks.
Protozoal origin. Leishmaniasis affects about two million people worldwide annually. It is caused by the protozoan Leishmania and is spread by bites of infected sandflies. Among the different types of leishmaniasis, the most common are cutaneous leishmaniasis (marked by skin sores) and visceral leishmaniasis, also known as kala azar, which is characterized by bouts of fever, splenomegaly (an enlarged spleen), weight loss, and anemia.
Chagas’ disease has an acute phase and a chronic phase. In the acute phase, the symptoms, if any, are mild and may include fever, fatigue, rash, aches, swollen spleen, and lymph nodes. Chronic symptoms can include irregular heartbeat, cardiomyopathy (an enlarged heart), and congestive heart failure.
Malaria is a greater threat in the tropics, but every year, about fifteen hundred cases are reported in the United States. Malaria is characterized by symptoms such as fever with chills, anemia, nausea, headaches, vomiting, and a feeling of malaise. Usually, malaria is treatable, but a certain Plasmodium species, P. falciparum, can cause fatal cerebral malaria if left untreated.
Helminthic origin. Ascariasis, if mild, may not produce symptoms. However, moderate to heavy infestation may lead to vomiting, diarrhea, bloody stools, abdominal pain, and worm in the stool.
Lymphatic filariasis (elephantiasis), seen only in a small percentage of infected persons, will produce swelling in the limbs from lymphedema, an accumulation of lymph caused by lymphatic system dysfunction. Because the lymphatic system is part of the body’s immune defense, affected people are more prone to bacterial infections in the skin and lymph. This leads to chronic hardening and thickening of the skin surface (in which skin texture resembles those of elephants) called elephantiasis.
Persons who have an arthropod-borne disease are diagnosed based on clinical signs and symptoms. For some diseases, such as bubonic plague, characteristic signs help with a definitive diagnosis. Lyme disease is often diagnosed by the classic symptom of a circular skin rash called erythema nigrans. Because not all persons who have been exposed to arthropods such as ticks and who suspect they might develop Lyme disease will get a skin rash, doctors will often order laboratory tests such as ELISA (enzyme linked immunosorbent assay) to test for either an antigen (unique to the pathogen) or an antibody (made in the host to fight the antigen). Persons who test positive in the ELISA test are then recommended for the Western blot assay, which is much more specific.
In case of an arboviral infection, once the preliminary diagnosis has been made on the basis of clinical signs, cerebrospinal fluid or serum is tested to check levels of virus-specific IgM (immunoglobulin M) and other neutralizing antibodies. Occasionally, in severe cases, specialized tests such as immunohistochemistry and virus cultures with tissue samples isolated from the affected individual are used. Diagnostic procedures that involve PCR (polymerase chain reaction) are also widely used because they offer a simple and sensitive method for confirming the diagnosis.
For diagnosing protozoan arthropod-borne diseases such as malaria, health care specialists will use techniques that include a microscopic examination of the blood to check for the presence of the malarial parasite and an RDT (rapid diagnostic test) to look for the presence of antigens borne by a malarial parasite. The RDT is usually available in dipstick or cassette format and, therefore, results can be obtained within a few minutes.
For persons who have symptoms of arthropod-borne diseases of helminthic origin, diseases such as ascariasis, stool samples are tested for the presence of the illness-causing worm. In other diseases of the same family, such as filariasis, blood samples are collected at night and tested for the presence of microfilariae (larval forms of filaria). The blood sample has to be collected from the patient at night because the microfilariae are circulating in the patient’s blood at that time.
Arthropod-borne illness and disease have become points of major concern because, for most of these diseases, no easy treatments or vaccinations exist. Indeed, the treatments for certain vector-borne diseases can be difficult and dangerous. Diseases of bacterial origin, such as Lyme disease, are treated with antibiotics such as doxicillin and amoxicillin. If started in the early stages of illness, these antibiotics can completely cure the patient.
Arthropod-borne diseases of viral origin have no specific treatment, and treatment for protozoan diseases, such as malaria, involves the use of antimalarial drugs such as quinine and chloroquine; leishmaniasis is treated with drugs such as liposomal amphotericin B. Helminthic origin diseases are treated with drugs such as albendazole, mebendazole, or a yearly dose of diethycarbamazine, which kills circulating microfilariae.
Because these diseases are spread by insect bites, the simplest measures to prevent illness are to block insects from accessing the host and to practice good sanitation and hygiene. Therefore, one should wear covered clothing such as long-sleeve shirts and pants when outdoors and should use insect repellants such as permethrin or those with NN-diethyl metatoluamide, or DEET (about 20 to 30 percent). Similarly, to minimize the chance of insect bites, one should avoid wooded areas in the late evening or nighttime hours and should routinely check clothes and accessories for ticks and fleas after returning indoors. In the case of diseases such as malaria, additional preventive measures include using insecticide-sprayed sleeping nets (around the bed) and using chemoprophylaxis (taking malaria-prevention drugs over time). Finally, for diseases such as ascariasis, helpful prevention strategies include avoiding soil that is infested with the worm that causes the disease and washing and peeling all fruits and vegetables. A proper understanding of these preventive measures is especially important because many of these diseases are included on the World Health Organization’s list of emerging infectious diseases.
Ashford, R. W., and W. Crewe. The Parasites of “Homo sapiens”: An Annotated Checklist of the Protozoa, Helminths, and Arthropods for Which We Are Home. 2d ed. New York: Taylor & Francis, 2003. An examination of the parasites that are familiar to the human body.
Atkinson, P. W., ed. Vector Biology, Ecology, and Control. New York: Springer Science, 2010. This book is a good source for the reader who needs a detailed study of vectors and latest methods for effective vector control.
Busvine, James R. Disease Transmission by Insects: Its Discovery and Ninety Years of Effort to Prevent It. New York: Springer, 1993. This book provides a historical perspective on the evolution of and progress in the study of arthropod-borne diseases.
Jong, Elaine C., and Russell McMullen, eds. Travel and Tropical Medicine Manual. 4th ed. Philadelphia: Saunders/Elsevier, 2008. A useful reference manual with advice on preventing, evaluating, and managing diseases that can be acquired in tropical environments and countries outside the United States.
Marquardt, William C., ed. Biology of Disease Vectors. 2d ed. New York: Academic Press/Elsevier, 2005. This textbook is geared to graduate students and researchers, but most of the information is accessible to general readers.
O’Hanlon, Leslie Harris. “Tinkering with Genes to Fight Insect-Borne Disease: Researchers Create Genetically Modified Bugs to Fight Malaria, Chagas’, and Other Diseases.” The Lancet 363 (April 17, 2004): 1288. Discusses genetic engineering techniques and their anticipated results.
Tortora, Gerard J., Berdell R. Funke, and Christine L. Case. Microbiology: An Introduction. 10th ed. San Francisco: Benjamin Cummings, 2010. A great reference for exploring the microbial world. Provides readers with an appreciation of the pathogenicity and usefulness of microorganisms.